March 10,
2013 "Information
Clearing House"
- "NYT"
-- Global temperatures are warmer than at any time in at least
4,000 years, scientists reported Thursday, and over the coming
decades are likely to surpass levels not seen on the planet
since before the last ice age.

Previous research had extended back roughly 1,500 years, and
suggested that the rapid temperature spike of the past century,
believed to be a consequence of human activity, exceeded any
warming episode during those years. The new work confirms that
result while suggesting the modern warming is unique over a
longer period.

Even if the temperature
increase from human activity that is projected for later this
century comes out on the low end of estimates, scientists said,
the planet will be at least as warm as it was during the warmest
periods of the modern geological era, known as the Holocene, and
probably warmer than that.

That epoch began about
12,000 years ago, after changes in incoming sunshine caused vast
ice sheets to melt across the Northern Hemisphere. Scientists
believe the moderate climate of the Holocene set the stage for
the rise of human civilization roughly 8,000 years ago and
continues to sustain it by, for example, permitting a high level
of food production.

In the
new research, scheduled for publication on Friday in the
journal Science, Shaun Marcott, an earth scientist at
Oregon State University, and his colleagues compiled the
most meticulous reconstruction yet of global temperatures over
the past 11,300 years, virtually the entire Holocene. They used
indicators like the distribution of microscopic,
temperature-sensitive ocean creatures to determine past climate.

Like previous such
efforts, the method gives only an approximation. Michael E.
Mann, a researcher at Pennsylvania State University who is an
expert in the relevant techniques but was not involved in the
new research, said the authors had made conservative data
choices in their analysis.

“It’s another important
achievement and significant result as we continue to refine our
knowledge and understanding of
climate change,” Dr. Mann said.

Though the paper is the
most complete reconstruction of global temperature, it is
roughly consistent with previous work on a regional scale. It
suggests that changes in the amount and distribution of incoming
sunlight, caused by wobbles in the earth’s orbit, contributed to
a sharp temperature rise in the early Holocene.

The climate then
stabilized at relatively warm temperatures about 10,000 years
ago, hitting a plateau that lasted for roughly 5,000 years, the
paper shows. After that, shifts of incoming sunshine prompted a
long, slow cooling trend.

The cooling was
interrupted, at least in the Northern Hemisphere, by a fairly
brief spike during the Middle Ages, known as the Medieval Warm
Period. (It was then that the Vikings settled Greenland, dying
out there when the climate cooled again.)

Scientists say that if
natural factors were still governing the climate, the Northern
Hemisphere would probably be destined to freeze over again in
several thousand years. “We were on this downward slope,
presumably going back toward another ice age,” Dr. Marcott said.

During the long climatic
plateau of the early Holocene, global temperatures were roughly
the same as those of today, at least within the uncertainty of
the estimates, the new paper shows. This is consistent with a
large body of past research focused on the Northern Hemisphere,
which showed a distribution of ice and vegetation suggestive of
a relatively warm climate.

The modern rise that has
recreated the temperatures of 5,000 years ago is occurring at an
exceedingly rapid clip on a geological time scale, appearing in
graphs in the new paper as a sharp vertical spike. If the rise
continues apace, early Holocene temperatures are likely to be
surpassed within this century, Dr. Marcott said.

Dr. Mann pointed out that
the early Holocene temperature increase was almost certainly
slow, giving plants and creatures time to adjust. But he said
the modern spike would probably threaten the survival of many
species, in addition to putting severe stresses on human
civilization.

“We and other living
things can adapt to slower changes,” Dr. Mann said. “It’s the
unprecedented speed with which we’re changing the climate that
is so worrisome.”

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